Light emitting diodes are a widely used illumination source for obtaining accurate color measurements. However, thermal control presents a significant challenge to obtaining accurate measurement in LED-based measurement applications. Specifically, when an LED is activated for an extended time, thermal accumulation on the P-N junction will cause a spectrum change in the light emitted by the LED.
This problem is overcome in dual channel spectrophotometers through the use of a reference channel that monitors the intensity variation of the LED source at each wavelength, and thus the spectrum change of the LED is not a concern. For example, the Datacolor 45G spectrophotometer manufactured by Datacolor Inc. of Lawrenceville, N.J., uses multiple LEDs to generate spectrally-uniform illumination to provide accurate color measurement. However, these LEDs are thermally sensitive. Temperature build-up at LED P-N junctions causes a spectrum change in the emitted light. In order to compensate for this spectrum change the 45G uses a reference channel to monitor the intensity change at each wavelength. The cost of such a reference channel is usually high.
Therefore, what is needed is a simple, inexpensive way to monitor and compensate the spectrum change of LED light at each wavelength.